Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 31: Which among the following amino acids absorbs UV light?

A. Leucine
B. Lysine
C. Tyrosine (Correct Answer)
D. Valine

Explanation: ### Explanation **Correct Option: C. Tyrosine** The ability of proteins to absorb Ultraviolet (UV) light is primarily due to the presence of **Aromatic Amino Acids**. These amino acids contain a conjugated double-bond system in their side chains (aromatic rings), which allows them to absorb light in the UV spectrum, typically at a wavelength of **280 nm**. * **Tryptophan:** The strongest absorber of UV light (highest molar extinction coefficient). * **Tyrosine:** The second most potent absorber. * **Phenylalanine:** Absorbs UV light at a lower wavelength (approx. 260 nm) and is much less efficient than the other two. In clinical and laboratory settings, measuring absorbance at 280 nm is a standard technique for estimating protein concentration in a solution. **Why Incorrect Options are Wrong:** * **A. Leucine:** An aliphatic, branched-chain amino acid. It lacks an aromatic ring and does not absorb UV light at 280 nm. * **B. Lysine:** A basic, positively charged amino acid. It does not possess a conjugated ring system. * **D. Valine:** An aliphatic, branched-chain amino acid. Like Leucine, it lacks the structural requirements for UV absorption. **High-Yield Facts for NEET-PG:** 1. **Order of UV Absorption (at 280 nm):** Tryptophan > Tyrosine > Phenylalanine. 2. **Beer-Lambert Law:** This principle is used to calculate protein concentration based on UV absorbance. 3. **DNA/RNA Absorption:** Nucleic acids absorb UV light at **260 nm** (due to purine and pyrimidine bases). The **260/280 ratio** is used to check the purity of DNA/RNA samples (a ratio of ~1.8 is considered pure for DNA). 4. **Ninhydrin Test:** Most amino acids give a purple/Ruhemann's purple color; however, **Proline** gives a yellow color.

Question 32: Which of the following amino acids shows a tendency to form a left-handed helix?

A. Cysteine
B. Glycine (Correct Answer)
C. Arginine
D. Histidine

Explanation: **Explanation:** The correct answer is **Glycine**. **1. Why Glycine is Correct:** In protein biochemistry, the alpha-helix is typically right-handed due to the L-configuration of amino acids. However, **Glycine** is unique because it is the only achiral amino acid (its side chain is a single hydrogen atom). This lack of a bulky side chain provides Glycine with exceptional conformational flexibility, allowing it to occupy regions of the **Ramachandran plot** that are sterically forbidden for other amino acids. Specifically, Glycine can adopt the dihedral angles required to form a **left-handed helix**, a structure that would cause steric hindrance (clashing) between the side chains and the backbone in any other amino acid. **2. Why the Other Options are Incorrect:** * **Cysteine, Arginine, and Histidine:** These are all **L-amino acids** with distinct side chains (R-groups). Due to steric interference between the side chain and the carbonyl oxygen of the peptide backbone, these amino acids strongly prefer the right-handed helical conformation. A left-handed helix formed by these residues is energetically unstable and rarely occurs in nature. **3. NEET-PG High-Yield Facts:** * **Glycine as a "Helix Breaker":** While it can form a left-handed helix, Glycine often acts as a "helix breaker" in standard right-handed alpha-helices because its high flexibility makes the rigid helical structure entropically unfavorable. * **Proline:** Another "helix breaker" because its rigid cyclic structure lacks the NH group necessary for hydrogen bonding. * **Collagen Triple Helix:** Glycine is essential here; it occurs at every third position (**Gly-X-Y**) because only Glycine is small enough to fit into the crowded central core of the triple helix. * **Ramachandran Plot:** Glycine shows the widest range of permissible Phi ($\phi$) and Psi ($\psi$) angles.

Question 33: Which of the following organelles are involved in the formation of N-glycosylated products?

A. Golgi apparatus and RER (Correct Answer)
B. Nucleolus
C. SER
D. RER and Nucleolus

Explanation: **Explanation:** N-glycosylation is a critical post-translational modification where a carbohydrate chain is attached to the nitrogen atom of an **Asparagine (Asn)** residue within the consensus sequence Asn-X-Ser/Thr. This process is a coordinated effort between two specific organelles: 1. **Rough Endoplasmic Reticulum (RER):** This is where N-glycosylation **begins**. A pre-formed oligosaccharide (attached to a **Dolichol** phosphate lipid carrier) is transferred en bloc to the nascent polypeptide chain as it enters the RER lumen. 2. **Golgi Apparatus:** The glycoprotein then moves to the Golgi, where the initial carbohydrate chain undergoes extensive **processing and modification** (trimming and addition of sugar residues) to form complex or high-mannose glycans. **Analysis of Incorrect Options:** * **Nucleolus:** Primarily involved in ribosomal RNA (rRNA) synthesis and ribosome biogenesis; it plays no role in protein glycosylation. * **SER (Smooth ER):** Involved in lipid synthesis, steroidogenesis, and detoxification (Cytochrome P450 system), but lacks the ribosomes and machinery required for the initial steps of N-glycosylation. * **RER and Nucleolus:** While RER is correct, the inclusion of the nucleolus makes this option incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Dolichol Phosphate:** The essential lipid carrier inhibited by **Tunicamycin**. * **I-Cell Disease (Mucolipidosis II):** A deficiency in the enzyme *N-acetylglucosamine-1-phosphotransferase* in the Golgi. This leads to a failure to tag enzymes with **Mannose-6-Phosphate**, causing lysosomal enzymes to be secreted extracellularly instead of being routed to lysosomes. * **O-glycosylation:** Unlike N-glycosylation, O-linked glycosylation (attachment to Serine/Threonine) occurs **exclusively in the Golgi apparatus**.

Question 34: What are clusters of twisted strands of β sheet called?

A. Amphipathic helices
B. β barrels (Correct Answer)
C. Loops and bends
D. Supersecondary structures

Explanation: **Explanation:** The correct answer is **β barrels**. This structure is a large, closed β-sheet that twists and coils to form a closed, cylinder-like structure. In this arrangement, the first strand is hydrogen-bonded to the last strand, creating a stable, pore-like architecture. These are commonly found in transmembrane proteins (porins) where the exterior is hydrophobic (interacting with lipids) and the interior is hydrophilic (allowing water/solute passage). **Analysis of Options:** * **Amphipathic helices (A):** These are α-helices with one hydrophobic face and one hydrophilic face. While important for protein-lipid interactions, they are not composed of twisted β-sheets. * **Loops and bends (C):** These are non-repetitive secondary structures that connect α-helices and β-sheets. They allow the polypeptide chain to reverse direction but do not form large "clusters" or "barrels." * **Supersecondary structures (D):** This is a broad category (also called motifs) that includes structures like β-α-β or Greek keys. While a β-barrel *is* a type of supersecondary structure, the question specifically describes the "clusters of twisted strands" forming a specific shape, for which "β barrel" is the most precise anatomical description. **NEET-PG High-Yield Pearls:** * **Porins:** Found in the outer membranes of Gram-negative bacteria and mitochondria, porins are the classic example of β-barrels. * **Green Fluorescent Protein (GFP):** A famous example of a "β-can" or barrel structure used in molecular biology. * **Stability:** The β-barrel is exceptionally stable due to the extensive hydrogen bonding between all adjacent strands, making it resistant to denaturation. * **Beta-turns:** Often contain **Proline** (induces a kink) and **Glycine** (small and flexible), and are usually stabilized by hydrogen bonds between the 1st and 4th residues.

Question 35: Edema occurs when plasma protein level is below what value?

A. 5 mg/dl (Correct Answer)
B. 15 mg/dl
C. 10 mg/dl
D. 20 mg/dl

Explanation: **Explanation:** The correct answer is **5 mg/dl** (often expressed as **5 g/dL** in clinical practice; note that in many exams, these units are used interchangeably despite the technical difference). **Underlying Concept:** The primary force keeping fluid within the intravascular compartment is the **Plasma Colloid Osmotic Pressure (Oncotic Pressure)**, which is predominantly maintained by **Albumin**. According to Starling’s Law, fluid movement depends on the balance between hydrostatic pressure (pushing fluid out) and oncotic pressure (pulling fluid in). Normal total plasma protein levels range from **6–8 g/dL**. When the total protein level drops below **5 g/dL** (or albumin falls below **2.5 g/dL**), the oncotic pressure becomes insufficient to counteract hydrostatic pressure. This leads to the extravasation of fluid into the interstitial space, resulting in **edema**. **Analysis of Options:** * **A (5 mg/dl/g/dl):** This is the critical threshold. Below this level, the "tipping point" of Starling forces is reached, leading to clinically evident edema. * **B, C, and D:** These values (10, 15, 20) are significantly higher than the physiological threshold. While a patient with 10 g/dL would be hyperproteinemic (e.g., in Multiple Myeloma), they would not develop edema due to low oncotic pressure. **Clinical Pearls for NEET-PG:** 1. **Albumin’s Role:** Albumin contributes roughly **70–80%** of the total plasma oncotic pressure due to its high concentration and small molecular weight. 2. **Hypoproteinemic States:** Common causes include **Nephrotic Syndrome** (protein loss), **Cirrhosis** (decreased synthesis), and **Kwashiorkor** (malnutrition). 3. **Reverse Trend:** In conditions like Multiple Myeloma, total protein increases (hyperglobulinemia), but the **A:G ratio (Albumin:Globulin)** reverses (normal is 1.2:1 to 2:1).

Question 36: The property of proteins to absorb ultraviolet rays of light is due to which component?

A. Peptide bond
B. Imino group
C. Disulfide bond
D. Aromatic amino acid (Correct Answer)

Explanation: **Explanation:** The ability of proteins to absorb ultraviolet (UV) light is primarily attributed to the presence of **aromatic amino acids**, specifically **Tryptophan, Tyrosine, and Phenylalanine**. These amino acids contain conjugated double bonds in their ring structures (delocalized pi-electron systems) that resonate when exposed to UV radiation. * **Tryptophan** is the strongest absorber, followed by **Tyrosine**. * Proteins show a characteristic absorption peak at **280 nm**. This property is routinely used in laboratories to quantify protein concentration in a solution using spectrophotometry (Beer-Lambert Law). **Analysis of Incorrect Options:** * **Peptide bond:** While peptide bonds do absorb UV light, they do so in the "far-UV" range (approximately **190–210 nm**). This is not the standard wavelength used for protein quantification. * **Imino group:** This refers to Proline. While it affects protein folding and secondary structure (helix breaker), it does not contribute significantly to UV absorption at 280 nm. * **Disulfide bond:** These covalent bonds (between two Cysteine residues) stabilize tertiary and quaternary structures but are not responsible for the characteristic 280 nm UV absorption. **High-Yield Clinical Pearls for NEET-PG:** * **Absorption Peak:** Proteins = 280 nm; Nucleic Acids (DNA/RNA) = 260 nm. * **Purity Check:** The 260/280 ratio is used to estimate the purity of DNA samples (a ratio of ~1.8 is considered pure DNA). * **Fluorescence:** Tryptophan is also the major contributor to the intrinsic fluorescence of proteins. * **Mnemonic:** "Aromatic rings absorb the rays" (Tryptophan > Tyrosine > Phenylalanine).

Question 37: The study of the multiplication of proteins in a disease process is known by which of the following terms?

A. Proteomics (Correct Answer)
B. Genomics
C. Glycomics
D. Nucleomics

Explanation: ### Explanation **Correct Answer: A. Proteomics** **1. Why Proteomics is Correct:** Proteomics is the large-scale study of **proteomes**, which refers to the entire set of proteins expressed by a genome, cell, tissue, or organism at a given time. In the context of a disease process, proteomics involves studying the multiplication, expression levels, post-translational modifications, and interactions of proteins. Since proteins are the functional molecules that execute biological processes, analyzing their changes is crucial for understanding disease pathogenesis, identifying biomarkers, and developing targeted therapies. **2. Why Other Options are Incorrect:** * **B. Genomics:** This is the study of the complete set of DNA (genes) within an organism. While it provides the blueprint, it does not account for dynamic changes in protein levels or functions during a disease. * **C. Glycomics:** This refers to the comprehensive study of **glycans** (sugars/carbohydrates) and their structures and functions in biological systems. * **D. Nucleomics:** This is a less common term referring to the study of the structure and function of the cell nucleus and its components (often used interchangeably with aspects of genomics or transcriptomics). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Central Dogma:** DNA (Genomics) → RNA (Transcriptomics) → Protein (**Proteomics**). * **Dynamic Nature:** Unlike the genome, which is relatively constant, the proteome is highly dynamic and changes in response to external stimuli or disease states. * **Clinical Application:** Many diagnostic markers used in clinical practice (e.g., Troponin I for MI, PSA for Prostate Cancer) are products of proteomic identification. * **Technique:** **Mass Spectrometry (MS)** and **2D-Gel Electrophoresis** are the gold-standard techniques used in proteomic analysis.

Question 38: What is the isoelectric point?

A. The pH at which a molecule carries no net electrical charge
B. The pH at which a molecule exists as a zwitterion
C. The pH at which a protein precipitates
D. All of the above (Correct Answer)

Explanation: The **Isoelectric Point (pI)** is the specific pH at which a molecule, such as an amino acid or protein, carries a **net electrical charge of zero**. This concept is fundamental in biochemistry for understanding protein behavior and separation techniques. ### Why "All of the Above" is Correct: 1. **Option A (Net Zero Charge):** By definition, at the pI, the number of positive charges (from protonated amino groups) exactly equals the number of negative charges (from deprotonated carboxyl groups). 2. **Option B (Zwitterion):** At this pH, the molecule exists as a **zwitterion** (dipolar ion). While it contains both positive and negative functional groups, they cancel each other out, resulting in electrical neutrality. 3. **Option C (Precipitation):** Proteins are most soluble when they are charged because they repel each other and interact with water. At the pI, the lack of net charge leads to **minimum electrostatic repulsion**, causing protein molecules to aggregate and precipitate out of solution. ### High-Yield NEET-PG Clinical Pearls: * **Electrophoresis:** At a pH **above** its pI, a protein is negatively charged and moves toward the **Anode (+)**. At a pH **below** its pI, it is positively charged and moves toward the **Cathode (-)**. At the pI, there is no mobility. * **Calculation:** For simple amino acids, $pI = (pK_1 + pK_2) / 2$. * **Clinical Application:** **Isoelectric Focusing (IEF)** is a laboratory technique used to separate proteins (like hemoglobin variants) based on their specific pI values along a pH gradient. * **Case Scenario:** In the stomach (low pH), most proteins are below their pI and carry a net positive charge.

Question 39: What is the most abundant protein found in the human body?

A. Globulin
B. Albumin
C. Collagen (Correct Answer)
D. Hemoglobin

Explanation: **Explanation:** **Correct Answer: C. Collagen** Collagen is the most abundant protein in the human body, accounting for approximately **25% to 35% of the total protein mass**. It is a structural fibrous protein found primarily in the extracellular matrix of connective tissues, including skin, bones, tendons, cartilage, and blood vessels. Its high abundance is due to its critical role in providing tensile strength and structural integrity to almost every organ system. **Analysis of Incorrect Options:** * **A. Globulin:** These are a group of globular proteins (like immunoglobulins) found in the blood. While vital for immunity and transport, they do not match the sheer structural mass of collagen. * **B. Albumin:** This is the most abundant **plasma** protein (making up ~60% of serum proteins). Students often confuse "most abundant protein in the body" with "most abundant protein in the plasma." * **D. Hemoglobin:** This is the most abundant protein within **red blood cells** and is essential for oxygen transport, but its total body mass is less than that of collagen. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** Collagen is a triple helix composed of three polypeptide chains. Every third amino acid is **Glycine** (the smallest amino acid, allowing tight packing). * **Post-translational modification:** Hydroxylation of Proline and Lysine requires **Vitamin C** (Ascorbic acid). Deficiency leads to **Scurvy**. * **Types to Remember:** * Type I: Bone, Skin, Tendon (90% of body collagen). * Type II: Cartilage ("Car-two-lage"). * Type III: Reticular fibers, Blood vessels (Deficient in Vascular Ehlers-Danlos). * Type IV: Basement membrane ("Under the floor").

Question 40: The amino acid sequence of human chorionic gonadotropin (hCG) is approximately 80% similar to which of the following hormones?

A. Follicle-stimulating hormone (FSH)
B. Luteinizing hormone (LH) (Correct Answer)
C. Gonadotropin-releasing hormone (GnRH)
D. None of the above

Explanation: **Explanation:** Human Chorionic Gonadotropin (hCG) belongs to the family of **glycoprotein hormones**, which also includes Luteinizing Hormone (LH), Follicle-Stimulating Hormone (FSH), and Thyroid-Stimulating Hormone (TSH). All these hormones are heterodimers consisting of two subunits: **Alpha (α) and Beta (β).** 1. **The Alpha (α) Subunit:** This is identical in all four hormones (hCG, LH, FSH, and TSH). 2. **The Beta (β) Subunit:** This provides biological and immunological specificity. **Why LH is the correct answer:** The β-subunit of hCG shares approximately **80% sequence homology** with the β-subunit of LH. Both hormones bind to the same receptor (LH/hCG receptor). This structural similarity allows hCG to mimic the action of LH, which is crucial for maintaining the corpus luteum and progesterone production during early pregnancy. **Why other options are incorrect:** * **FSH:** While FSH shares the same α-subunit, its β-subunit has significantly lower homology with hCG compared to LH. * **GnRH:** This is a small decapeptide (10 amino acids) produced by the hypothalamus. It is not a glycoprotein and bears no structural resemblance to hCG. **High-Yield NEET-PG Pearls:** * **Biological Half-life:** hCG has a much longer half-life (~24–36 hours) than LH (~20–60 minutes) due to a highly glycosylated C-terminal tail on its β-subunit. * **Clinical Application:** Because of the identical α-subunits, pregnancy tests and immunoassay kits specifically target the **β-subunit of hCG** to avoid cross-reactivity with LH, FSH, or TSH. * **Therapeutic Use:** Due to its LH-like action and longer half-life, hCG is used clinically to trigger ovulation in infertility treatments (acting as an "LH surge").

Practice by Chapter

Amino Acids: Structure and Properties

Practice Questions

Peptide Bond Formation

Practice Questions

Primary Structure of Proteins

Practice Questions

Secondary Structure of Proteins

Practice Questions

Tertiary and Quaternary Structures

Practice Questions

Protein Folding and Chaperones

Practice Questions

Protein Domains and Motifs

Practice Questions

Structure-Function Relationships

Practice Questions

Hemoglobin and Myoglobin

Practice Questions

Collagen and Elastin

Practice Questions

Albumin and Plasma Proteins

Practice Questions

Post-Translational Modifications

Practice Questions

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